Apparatus and method of hot bulge forming, and product formed by hot bulge forming

ABSTRACT

In a hot bulge forming apparatus  1 , a preheated tubular material  10   b  is disposed in a cavity  33 A defined by dies  21 A,  31 A, air is supplied into an interior of the tubular material  10   b  so that the tubular material  10   b  is pressed against cavity surfaces  211 A,  311 A by the pressure of the air to thereby form the tubular material  10   b  into a shape defined by the cavity  33 A. Thereafter, the resulting tubular material  10   c  is disposed in a cavity  33 B defined by dies  21 B,  31 B, and air is supplied into an interior of the tubular material  10   c  so that the tubular material  10   c  is pressed against cavity surfaces  211 B,  311 B by the pressure of the air to thereby form the tubular material  10   c  into a shape defined by the cavity  33 B while the tubular material  10   c  is cooled by the cavity surfaces  211 B,  311 B. Projecting portions  331  each having an arc-shaped section are formed at portions of the cavity surfaces  211 A,  311 A against which lengthwise end sides of the tubular material  10   b  are pressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hot bulge forming apparatus, a hot bulge forming method and a product formed through hot bulge forming. More particularly, the invention relates to a hot bulge forming apparatus and method in which a preheated tubular workpiece is disposed in a cavity defined between dies, a fluid is supplied into the tubular workpiece in the cavity to press the workpiece against cavity surfaces of the dies by virtue of the pressure of the fluid so as to form the tubular workpiece, and thereafter the formed workpiece is cooled in the dies, and a product formed by the hot bulge forming apparatus according to the hot bulge forming method.

2. Related Art

Conventionally, there has been known a hot bulge forming process in which high-pressure air is supplied into a cavity defined between dies so as to form a tubular workpiece disposed in the cavity into a desired configuration.

Specifically, in this hot bulge forming process, for example, a tubular workpiece is preheated, and the preheated tubular workpiece is disposed between a pair of dies. Next, the dies are clamped together while the workpiece is restrained at both lengthwise ends thereof, and high-pressure air is supplied into a cavity defined between the clamped dies so that the workpiece is pressed against cavity surfaces of the dies. Thereafter, this state is maintained for a certain length of time to cool the workpiece in the dies. Then, the dies are opened to release the internal pressure therein, and the formed workpiece is removed from the dies (for example, refer to US2005/0029714).

Incidentally, when the internal pressure is released, the force is reduced which presses the inner side of the workpiece, and therefore, the workpiece largely shrinks. As this occurs, a degree of shrinkage at both lengthwise end sides of the workpiece is larger than a degree of shrinkage at a lengthwise central side thereof. Because of this, the lengthwise central side is dragged by the shrinkage at both the lengthwise end sides and a part of the lengthwise central side dents, generating a dent. In particular, as is shown in FIG. 10, in the case of a sectional shape of a lengthwise central side of a workpiece 110 being rectangular, a rigidity of longer side portions 111 is lower than a rigidity of shorter side portions, and therefore, the longer side portions largely dent.

This kind of phenomenon is occurred by the following reason.

When forming bulges, since high-pressure air is supplied in such a state that the workpiece is restrained at both the lengthwise end sides thereof, the lengthwise central side of the workpiece is pressed against the cavity more strongly by the pressure of the supplied air than the lengthwise end sides are. Therefore, the lengthwise central side of the workpiece is cooled more quickly than the lengthwise end sides thereof, and cooling is progressed further at the lengthwise central side than at the lengthwise end sides before the internal pressure is released. Consequently, when the internal pressure is released, the workpiece shrinks more largely at the lengthwise end sides than at the lengthwise central side, and the lengthwise central side of the workpiece is dragged by the shrinkage at both the lengthwise end sides to dent, generating a dent thereat.

SUMMARY OF THE INVENTION

One or more embodiments of the invention provide a hot bulge forming apparatus which can suppress a generation of a dent at a lengthwise central side of a workpiece.

In accordance with one or more embodiments of the invention, in a hot bulge forming apparatus (for example, a hot bulge forming apparatus 1, of an exemplary embodiment), a preheated tubular workpiece (for example, a tubular workpiece 10 b) is formed into a desired shape by disposing the preheated tubular workpiece in a cavity (for example, a cavity 33A) defined by first dies (for example, a lower die 21A, an upper die 31A), supplying a fluid (for example, air) into an interior of the workpiece so as to press the workpiece against cavity surfaces (for example, cavity surfaces 211A, 311A) of the first dies by virtue of a pressure of the fluid to form into a shape defined by the cavity surfaces, and thereafter, disposing the workpiece in a cavity (for example, a cavity 33B) of second dies (for example, a lower die 21B, an upper die 31B) and supplying a fluid into the interior of the workpiece so as to press the workpiece against cavity surfaces (for example, cavity surfaces 211B, 311B) of the second dies by virtue of a pressure of the fluid so that the workpiece is cooled by the cavity surfaces while being formed into a shape defined by the cavity surfaces. In the hot bulge forming apparatus projecting portions (for example, projecting portions 331) each having an arc-shaped section are formed at portions of the cavity surfaces of the first dies against which lengthwise end sides of the workpiece are pressed.

According to this structure, the projecting portions each having the arc-shaped section are formed at the portions of the cavity surfaces of the first dies against which the lengthwise end sides of the workpiece are pressed.

Consequently, when the workpiece is formed by the first dies, the projecting portions formed on the cavity surfaces are transferred on to the workpiece, whereby recess portions each having an arc-shaped section are formed at the lengthwise end sides of the workpiece. Thereafter, when the workpiece is formed into the shape defined by the second dies and is then cooled by the second dies so as to release an internal pressure in the cavity, the workpiece attempts to shrink more largely at the lengthwise side ends than at the lengthwise central side. Namely, a circumferential length of the workpiece at the lengthwise side ends attempts to decrease largely.

However, since the lengthwise end sides and the lengthwise central side of the workpiece are connected continuously, the shrinking deformation occurring at the lengthwise end sides of the workpiece is restrained by the lengthwise central side thereof. Consequently, the recess portion is tensioned, and the recess portion is deformed by the tensile force, whereby the curvature of the arc-like shape of the recess portion is reduced. Because of this, an attempt to reduce the circumferential length of the workpiece at the lengthwise end sides is suppressed.

As a result, the generation of a dent at the lengthwise central side of the workpiece can be suppressed which would otherwise occur by the lengthwise central side being dragged by the shrinkage occurring at the lengthwise end sides of the workpiece.

Moreover, in accordance with one or more embodiments of the invention, a hot bulge forming method includes the steps of: providing a recess portion having an arc-shaped section at a lengthwise end side of a tubular workpiece; and forming the tubular workpiece by disposing the tubular workpiece which is preheated and provided with the recessed portion in a cavity of a die and supplying a fluid into an interior of the workpiece so as to press the workpiece against a cavity surface of the die by virtue of a pressure of the fluid.

In addition, in accordance with one or more embodiments of the invention, a product which is formed through hot bulge forming is provided with a recess portion having an arc-shaped section at a lengthwise end side of the product.

According to the embodiments, when the workpiece is formed by the first dies, the projecting portions formed on the cavity surfaces are transferred on to the workpiece, whereby the recess portions each having the arc-shaped section are formed at the lengthwise end sides of the workpiece. Thereafter, the workpiece is formed and cooled by the second dies, and the internal pressure is released. As this occurs, the workpiece attempts to shrink more largely at the lengthwise end sides than at the lengthwise central side. Namely, the circumferential length of the workpiece attempts to decrease largely at the lengthwise end sides. However, since the lengthwise end sides of the workpiece are continuously connected to the lengthwise central side thereof, the shrinking deformation occurring at the lengthwise end sides is restrained by the lengthwise central side of the workpiece. Consequently, the recess portions are tensioned and are deformed by the tensile force, whereby the curvature of the arc-like shape of the recess portion is reduced. Therefore, the attempt to reduce the circumferential length at the lengthwise end sides of the workpiece is suppressed. As a result, the generation of a dent at the lengthwise central side of the workpiece can be suppressed which would otherwise occur by the lengthwise central side being dragged by the shrinkage at the lengthwise end sides of the workpiece.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing operations of a hot bulge forming apparatus according to an exemplary embodiment of the invention.

FIG. 2 ((a) portion to (d) portion) shows perspective views of a workpiece that is formed by the hot bulge forming apparatus.

FIG. 3 is a sectional view of a first bulge forming device which makes up the hot bulge forming apparatus.

FIG. 4 is a sectional view showing sections of dies of the first bulge forming device.

FIG. 5 is a sectional view of a second bulge forming device which makes up the hot bulge forming apparatus.

FIG. 6 is a sectional view showing sections of dies of the second bulge forming device.

FIG. 7 is a sectional view of a third bulge forming device which makes up the hot bulge forming apparatus.

FIG. 8 is a sectional view showing sections of dies of the third bulge forming device.

FIG. 9 ((a) portion and (b) portion) shows diagrams illustrating a deformation of the workpiece in the third bulge forming device.

FIG. 10 is a diagram illustrating a deformation of a workpiece according to a conventional example.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

An exemplary embodiment of the invention will be described by reference to the drawings.

FIG. 1 is a flow chart showing operations of a hot bulge forming apparatus 1 according to the exemplary embodiment of the invention.

FIG. 2 ((a) portion to (d) portion) shows perspective views of tubular materials 10 a to 10 d which represent workpieces which are formed at respective steps by the hot bulge forming apparatus 1.

The hot bulge forming apparatus 1 is designed to execute an energization heating process 2, a bulge forming process 3 and a squashing process 4 which constitutes a pre-forming process, and a section shaping process 5 which constitutes a final forming process sequentially in that order.

Specifically, in the energization heating process 2, a tubular material 10 a, which is made of an aluminum alloy and which extends substantially rectilinearly, is heated.

In the bulge forming process 3, portions of the tubular material 10 a which lie closer to ends thereof are expanded by a first bulge forming device 6 (refer to FIG. 3) so as to form the tubular material 10 a into a tubular material 10 b.

In the squashing process 4, a sectional shape of the tubular material 10 b is formed into a substantially oval shape and the tubular material 10 b is curved at an intermediate portion thereof by a second bulge forming device 7 (refer to FIG. 5) so as to form the tubular material 10 b into a tubular material 10 c.

In the section shaping process 5, a sectional shape of the tubular material 10 c is formed into a substantially rectangular shape by a third bulge forming device 8 (refer to FIG. 7) so as to form the tubular material 10 c into a tubular material 10 d.

FIG. 3 is a sectional shape showing a schematic configuration of the first bulge forming device 6. FIG. 4 is a sectional view of dies of the first bulge forming device 6.

The first bulge forming device 6 includes a lower die mechanism 20 which includes a lower die 21 which supports the tubular material 10 a, 10 b, an upper die mechanism 30 which includes an upper die 31 which holds the tubular material 10 a, 10 b together with the lower die 21 from above and below the tubular material 10 a, 10 b, a holding mechanism 40 for holding both end sides of the tubular material 10 a, 10 b, a pressing mechanism 50 for pressing both the end sides of the tubular material 10 a, 10 b in axial directions, an air supply mechanism 60 for supplying air into an interior of the tubular material 10 a, 10 b and heating units 70 for heating the lower die 21 and the upper die 31.

The lower die mechanism 20 includes the lower die 21 as a fixed die and a base 22 which supports the lower die 21. A cavity surface 211 is formed on the lower die 21.

The upper die mechanism 30 includes the upper die 31 as a movable die which is disposed above the lower die 21 so as to confront the lower die 21 and a lifting unit 32 for lifting up and down the upper die 31. A cavity surface 311 is formed on the upper die 31.

When the lifting unit 32 is driven to cause the upper die 31 to approach the lower die 21 so that the upper and lower dies are clamped together, a cavity 33 is defined by the cavity surface 311 of the upper die 31 and the cavity surface 211 of the lower die 21.

The holding mechanism 40 includes a pair of holders 41 which are provided at axial end sides of the tubular material 10 a, 10 b on the lower die 21 and reciprocating units 42 for causing the pair of holders 41 to reciprocate along an axial direction of the tubular material 10 a, 10 b.

The holder 41 has a substantially cylindrical shape.

The reciprocating units 42 cause the corresponding holders 41 to approach the tubular material 10 a, 10 b so as to fit on both the end sides of the tubular material 10 a, 10 b, whereby the tubular material 10 a, 10 b is held by the holders 41.

The pressing mechanism 50 includes a pair of pressing members 51 which are inserted individually into the pair of holders 41 and pressing units 52 for causing the pressing members 51 to reciprocate along the axial direction of the tubular material 10 a, 10 b.

The pressing units 52 cause the corresponding pressing members 51 to approach the tubular material 10 a, 10 b to be inserted individually into the corresponding holders 41 so as to press both ends of the tubular material 10 a, 10 b which is held by the holders 41, so that the tubular member 10 a, 10 b is compressed towards a center axis direction.

The air supply unit 60 includes air supply lines 61 which pass through the pair of pressing members 51 of the pressing mechanism to reach both the end sides of the tubular material 10 a, 10 b and an air pump, not shown, which supplies high-pressure air to these air supply lines 61.

The heating units 70 are incorporated in the lower die 21 and the upper die 31. A high-frequency current heating means, a heater heating unit and the like are raised for use as the heating units 70.

FIG. 5 is a sectional view showing a schematic configuration of the second bulge forming device 7. FIG. 6 is a sectional view of dies of the second bulge forming device 7.

The second bulge forming device 7 differs from the first bulge forming device 6 in that a cavity 33A defined by a cavity surface 311A of an upper die 31A and a cavity surface 211A of a lower die 21A has a different shape, in that an air supply unit 60 has a different construction, and in that the holding mechanism 40 and the pressing mechanism 50 are not provided but a restraining mechanism 80 is provided. The other configurations of the second bulge forming device 7 are similar to those of the first bulge forming device 6.

Namely, projecting portions 331 each having an arc-shaped section are formed at both lengthwise end sides of the cavity surface 311A of the upper die 31A and the cavity surface 211A of the lower die 21A, that is, portions of those cavity surfaces against which both the lengthwise end sides of the tubular material 10 b, 10 c are pressed.

In addition, the restraining mechanism 80 includes a pair of restraining beads 81 which are provided so as to hold the tubular material 10 b, 10 c on the lower die 21A from axial directions and reciprocating units 82 for causing the pair of restraining beads 81 to reciprocate along an axial direction of the tubular material 10 b, 10 c.

A recess portion 811 is formed in the restraining bead 81.

The reciprocating units 82 cause the corresponding restraining beads 81 to approach the tubular material 10 b, 10 c so as to allow both end sides of the tubular material 10 b, 10 c to fit in the corresponding recess portions 811, whereby the tubular material 10 b, 10 c is restrained at both the end sides thereof.

In addition, air supply lines 61A of an air supply unit 60 pass through the pair of restraining beads 81 to reach both the end sides of the tubular material 10 b, 10 c.

FIG. 7 is a sectional view showing a schematic configuration of the third bulge forming device 8. FIG. 8 is a sectional view of dies of the third bulge forming device 8.

The third bulge forming device 8 differs from the second bulge forming device 7 in that a cavity 33B defined by a cavity surface 311B of an upper die 31B and a cavity surface 211B of a lower die 21B has a different shape, and in that heating units 70B have a different configuration. The other configurations of the third bulge forming device 8 remain similar to those of the second bulge forming device 7.

Namely, no projecting portion 331 is formed on the cavity surface 311B of the upper die 31B and the cavity surface 211B of the lower die 21B.

In addition, a fluid heating means is used as the heating unit 70B.

Hereinafter, a bulge forming procedure by the hot bulge forming apparatus 1 will be described.

A bulge forming process includes an energization heating process, a bulge forming process, a squashing process and a section shaping process.

Firstly, the tubular material 10 a which is made of an aluminum alloy is heated to about 500° C. in the energization heating process.

Next, the bulge forming process is carried out. Specifically speaking, firstly, the dies 21, 31 are heated to about 500° C., that is, a recrystallization temperature of the tubular material 10 a or higher by the heating units 70.

Next, the tubular material 10 a heated in the way described above is disposed on the lower die 21.

Next, the lifting unit 32 of the upper die mechanism 30 is driven to lower the upper die 31, and the dies 21, 31 are clamped together.

Next, the reciprocating units 42 of the holding mechanism 40 are driven to cause the holders 41 to fit on the end sides of the tubular material 10 a so as to hold the tubular material 10 a.

Next, the pressing members 51 of the pressing mechanism 50 are driven, so that the ends of the tubular material 10 a which is held by the holders 41 are pressed in compressing directions by the pressing members 51. At the same time, the air pump of the air supply unit 60 is driven to supply high-pressure air into the cavity 33.

Then, hot bulge forming occurs in the tubular material 10 a in which the tubular material 10 a is allowed to bulge to follow the configuration of the cavity 33, whereby the tubular material 10 a is formed into the tubular material 10 b.

Next, the squashing process is carried out. Specifically, firstly, the dies 21A, 31A are heated to about 500° C. or the recrystallization temperature of the tubular material 10 b or higher by the heating units 70.

Next, the tubular material 10 b, which has been subjected to hot bulge forming, is transferred to be disposed on the lower die 21A by a known transfer means, not shown, while the heating state is maintained.

Next, the reciprocating units 82 of the restraining mechanism 80 are driven to cause the restraining beads 81 to fit on both the end sides of the tubular material 10 b.

In addition, the lifting unit 32 of the upper die mechanism 30 is driven to lower the upper die 31A, and the dies 21A, 31A are clamped together. At the same time, the air pump of the air supply unit 60 is driven to supply high-pressure air into the cavity 33A.

Then, the tubular material 10 b, which has been subjected to hot bulge forming, is hot squashed (at about 500° C.) to follow the configuration of the cavity 33A, whereby the tubular material 10 b is formed into the tubular material 10 c. As this occurs, the projecting portions 331 formed on the cavity surfaces 211A, 311A are transferred on to the tubular material 10 c, and recess portions 11 each having an arc-shaped section are formed at both lengthwise end sides of the tubular material 10 c (refer to (a) portion and (b) portion of FIG. 9).

Next, the section shaping process is carried out. Specifically speaking, firstly, the lower die 21B and the upper die 31B are heated to about 200° C. or the recrystallization temperature of the tubular material 10 c or lower by the heating units 70B.

Next, the tubular material 10 c, which has been subjected to the squashing operation, is rotated substantially 90° about the a center axis by a rotating means, not shown, and is thereafter transferred to be disposed on the lower die 21B by a known transfer means, riot shown.

Next, the reciprocating units 82 of the restraining mechanism 80 are driven to cause the restraining beads 81 to fit on both the end sides of the tubular material 10 c, whereby the tubular material 10 c is restrained at both the end sides thereof. In addition, the lifting unit 32 of the upper die mechanism 30 is driven to lower the upper die 31B, and the dies 21B, 31B are clamped together. At the same time, the air pump of the air supply unit 60 is driven to supply high-pressure air into the cavity 33B.

Then, the section of the tubular material 10 c, which has been subjected to the squashing operation, is shaped to follow the configuration of the cavity 33B, whereby the tubular material 10 c is formed into the tubular material 10 d.

In this section shaping process, since the temperatures of the dies 21B, 31B are about 200° C., the heat of the tubular material 10 c is conducted to the dies 21B, 31B, whereby the temperature of the tubular material 10 c is decreased. However, hot bulge forming is implemented to some extent.

Next, the temperatures of the dies 21B, 31B are held to the recrystallization temperature of the tubular material 10 d or lower, and the clamping state of the dies 21B, 31B is maintained for a certain length of time for cooling the tubular material 10 d. By doing this, the tubular material 10 d is cooled so as to allow a thermal shrinkage to occur therein, and thereafter, the internal pressure in the dies is released.

As this occurs, since the tubular material 10 d is restrained at both end portions thereof by the restraining beads 81, an axial thermal shrinkage of the tubular material 10 d is suppressed.

Here, as is shown in (a) portion of FIG. 9, one lengthwise end side of the tubular material 10 c on which recess portions 11 each having an arc-shaped section are formed is referred to as an end portion 12, and a lengthwise central side of the tubular material 10 c is referred to as a central portion 13.

When the tubular material 10 c is cooled by the dies 21B, 31B to release the internal pressure, the end portion 12 of the tubular material 10 c attempts to shrink more largely than the central portion 13 of the tubular material 10 c. Namely, as is shown in (b) portion of FIG. 9, a circumferential length of the end portion 12 of the tubular material 10 c attempts to decrease largely.

However, since the end portion 12 of the tubular material 10 c is continuously connected to the central portion 13 thereof, the shrinking deformation occurring at the end portion 12 of the tubular material 10 c is restrained by the central portion 13 of the tubular material 10 c. Consequently, the recess portions 11 are tensioned in directions indicated by white arrows in (b) portion of FIG. 9 and are deformed by the tensile force, whereby the curvature of the arc-like shape of the recess portion 11 is reduced. Consequently, the attempted reduction in circumferential length of the end portion 12 of the tubular material 10 c is suppressed.

According to the exemplary embodiment that has been described heretofore, the following advantage is provided. (1) The projecting portions 331 each having the arc-shaped section are formed at portions of the upper die 31A and the lower die 21A against which the lengthwise end sides of the tubular material 10 c are pressed.

Consequently, when forming the tubular material 10 by the dies 21A, 31A, the projecting portions 331 which are formed on the cavity surfaces 211A, 311A are transferred on to the tubular material 10 c, and the recess portions 11 each having the arc-shaped section are formed at both the lengthwise end sides of the tubular material 10 c. Thereafter, when the tubular material 10 c is formed by the dies 21B, 31B and is then cooled by the dies 21B, 31B to release the internal pressure therein, the recess portions 11 are tensioned and are then deformed by the tensile force, whereby the curvature of the arc-like shape of the recess portion 11 is decreased. Consequently, the attempted reduction in circumferential length of the end sides of the tubular material 10 c is suppressed. As a result, the generation of a dent at the lengthwise central side of the workpiece can be suppressed which would otherwise occur by the lengthwise central side being dragged by the shrinkage occurring at the lengthwise end sides of the workpiece.

While the invention has been described in connection with the specific exemplary embodiment, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the present invention.

For example, in the exemplary embodiment, while the projecting portions 331 are formed on the dies 21A, 31A of the second bulge forming device 8 and the projecting portions 331 are then transferred on to the tubular material 10 b when the squashing process is implemented, the invention is not limited thereto. Namely, projecting portions may be formed on the dies 21, 31 of the first bulge forming device 6 so that the projecting portions are transferred on to the tubular material 10 a when the bulge forming process is implemented.

In addition, in the exemplary embodiment, while the tubular material which takes the form of tubular materials 10 a to 10 d is described as being made of aluminum alloy, the invention is not limited thereto, and hence, the tubular material may be made of other metals.

In addition, in the exemplary embodiment, while air is supplied into the interior of the tubular material which takes the forms of tubular materials 10 a to 10 d by the air supply unit 60, the invention is not limited thereto, and hence, other fluids may be supplied thereinto.

It is aimed, therefore, to cover in the appended claims all such changes and modifications falling within the scope of the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1 hot bulge forming apparatus; 10 a to 10 d tubular material; 11 recess portion; 21A lower die (first die); 33A upper die (first die); 21B lower die (second die); 31B upper die (second die); 33A, 33B cavity; 211A, 211B, 311A, 311B cavity surface; 331 projecting portion. 

1. A hot bulge forming apparatus comprising: a first die; and a second die, wherein a preheated tubular workpiece is formed by disposing the preheated tubular workpiece in a cavity of the first die, supplying a fluid into an interior of the workpiece so as to press the workpiece against a cavity surface of the first die by virtue of a pressure of the fluid to form the workpiece into a shape defined by the cavity surface, and thereafter, disposing the workpiece in a cavity of the second die, and supplying a fluid into the interior of the workpiece so as to press the workpiece against cavity surface of the second die by virtue of a pressure of the fluid so that the workpiece is cooled by the cavity surface while being formed into a shape defined by the cavity surface, and wherein the hot bulge forming apparatus further comprises a projecting portion having an arc-shaped section and formed at a portion of the cavity surface of the first die against which a lengthwise end side of the workpiece is pressed.
 2. A hot bulge forming method comprising: providing a recess portion having an arc-shaped section at a lengthwise end side of a tubular workpiece; and forming the tubular workpiece by disposing the tubular workpiece which is preheated and provided with the recessed portion in a cavity of a die and supplying a fluid into an interior of the workpiece so as to press the workpiece against a cavity surface of the die by virtue of a pressure of the fluid.
 3. A product which is formed through hot bulge forming and comprises a recess portion having an arc-shaped section at a lengthwise end side of the product. 